Residential and some commercial heating, ventilating and air conditioning systems provide a thermostat to control the residential heating and cooling requirements. The thermostat is located in the zone to be controlled and typically includes mercury switches which change state in response to temperature conditions. The state is transmitted to a component of the refrigeration system in the form of a command to assume or to remain in that state. The commands transmitted by the thermostat do not contain any inherent information other than the refrigeration system component should be in a particular state. For example, typical commands transmitted by the thermostat might indicate that the heating or cooling equipment should be either on or off. Such a system is shown in U.S. Pat. No. 4,449,375 to Briccetti where a thermostat directly interfaces with a controller and selectively provides signals to the controller when fan or compressor operation is desired.
Most commercial HVAC systems provide an intelligent controller which is usually located in physical proximity to the compressor portion of the refrigeration system. This controller is provided with sensors extending into the zone to be controlled. These sensors provide such information as actual zone temperature, desired zone temperature in the form of setpoints for heating and cooling, and the desired mode of operation. The information is supplied as a quantitative value in either analog or digital form and the system controller thereafter makes decisions as to how the refrigeration system should be controlled.
The thermostat and the intelligent controller are basically incompatible because the thermostat transmits a desired state in the form of a command, while the intelligent controller requires information in the form of a quantitative value. Effectively the decision making process for the thermostat occurs in the zone to be controlled and the decisions are made by the thermostat using mercury switches or the equivalent. On the other hand, the decision making process for the controller occurs within the controller using sensory information from the zone to be controlled.
For a variety of reasons, it is desirable to allow thermostats to interface with intelligent controllers. Such reasons include upgrades to existing applications where an intelligent controller is added to the system but the original thermostats remain unchanged, and situations where a user is required to use a particular thermostat to meet either code or contractual requirements.